In contemporary optical networking applications, an essential building block is a device that can separate a multi-wavelength optical signal into multiple spectral channels and route the individual spectral channels into multiple output ports in a dynamically reconfigurable fashion, while exhibiting desired channel filtering characteristics (e.g., flat channel transfer functions and minimal channel crosstalk). It is further desired for such a device to provide “hitless” reconfiguration (i.e., no light is to be coupled to intermediate output ports when channel switching is taking place), short reconfiguration time, and channel power control capability (e.g., the optical power levels of the spectral channels coupled into the output ports are controlled at predetermined values).
Co-pending, commonly owned now U.S. Pat. No. 6,625,346 filed on Aug. 23, 2001 and incorporated herein by reference, discloses a free-space wavelength-separating-routing (WSR) apparatus. Depicted in FIG. 1A is an exemplary embodiment 100 of this WSR apparatus, comprising multiple input/output ports which may be an array of fiber collimators 110, providing an input port 110-1 and a plurality of output ports 110-2 through 110-N (N≧3); a wavelength-separator which in one form may be a diffraction grating 101; a beam-focuser in the form of a focusing lens 102; and an array of channel micromirrors 103. The WSR apparatus 100 may further comprise an array 120 of collimator-alignment mirrors 120-1 through 120-N, e.g., in a one-to-one correspondence with the input port 110-1 and output ports 110-2 through 110-N.
In operation, a multi-wavelength optical signal emerges from the input port 110-1, which may be directed onto the diffraction grating 101 by way of the input collimator-alignment mirror 120-1. The diffraction grating 101 angularly separates the multi-wavelength optical signal into multiple spectral channels. (For purposes of illustration and clarity, only three spectral channels are explicitly shown.) The focusing lens 102 in turn focuses the dispersed spectral channels into corresponding focused spots, impinging onto the channel micromirrors 103. Each channel micromirror receives a unique one of the spectral channels. The channel micromirrors 103 are individually controllable and movable (e.g., pivotable or rotatable), such that, upon reflection, the spectral channels are directed into selected ones of the output ports 110-2 through 110-N. Each output port may receive any number of the reflected spectral channels. The output collimator-alignment mirrors 120-2 through 120-N may further provide angular control of the reflected optical beams and thereby facilitate the coupling of the spectral channels into the respective output ports. A quarter-wave plate 104 may be additionally interposed between the diffraction grating 101 and the channel micromirrors 103 to mitigate any undesirable polarization-sensitive effect.
Depicted in FIG. 1B is a close-up view of the channel micromirrors 103 shown in the embodiment of FIG. 1A. By way of example, the channel micromirrors 103 are arranged in a one-dimensional array along the x-axis (i.e., the horizontal direction in the figure), so as to receive the focused spots of the spatially separated spectral channels in a one-to-one correspondence. (As in the case of FIG. 1A, only three spectral channels are illustrated, each represented by a converging beam.) The reflective surface of each channel micromirror lies in the x-y plane as defined in the figure and is movable, e.g., pivotable (or deflectable) about the x-axis. Each spectral channel, upon reflection, is deflected in the y-direction (e.g., downward) relative to its incident direction. The beam focuser 102 of FIG. 1A in turn translates the angular deflection into a corresponding spatial displacement, whereby the spectral channel is directed into the desired output port.
Thus, a distinct feature of the above WSR apparatus is that the motion of each channel micromirror is individually and continuously controllable, such that its position (e.g., pivoting angle) can be continuously adjusted. This enables each channel micromirror to direct its corresponding spectral channel to any one of multiple output ports.
As the demand for capacity grows, the spectral channels in optical networking applications may have increasingly narrower channel separation. A case in point may be DWDM (dense wavelength-division-multiplexing) applications, where the frequency spacing between two adjacent spectral channels is typically less than 100 GHz in the wavelength range of 1.3–1.6 μm. Accordingly, the channel micromirror array 103 in the WSR apparatus 100 of FIGS. 1A–1B may have to be equipped with increasingly smaller pitch (i.e., the separation between two adjacent micromirrors), in order to accommodate such applications. As a result, it may become difficult for the WSR apparatus 100 of FIGS. 1A–1B to maintain desired channel filtering and other performance characteristics. Fabrication of such narrow-pitch micromirror arrays would also be a formidable task.
A conventional approach for dealing with spectral channels with narrow channel spacing is to interleave the input multi-wavelength signal into two (e.g., “odd” and “even”) wavelength groups, prior to de-multiplexing each group into individual wavelengths (and performing subsequent routing). U.S. Pat. No. 6,181,849, for example, discloses an implementation of this approach that entails an optical interleaver operating in conjunction with two sets of wavelength multiplexing/demultiplexing units (e.g., waveguide gratings) along with switching/routing means. Ostensibly, this is an expensive and cumbersome undertaking.
In view of the foregoing, there is a need in the art for a new generation of dynamic wavelength routing devices that are particularly suitable for DWDM or other narrow-channel-spacing optical networking applications.